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Published on Web Date: July 01, 2010
r 2010 American Chemical Society 2165 DOI: 10.1021/jz100533t |J. Phys. Chem. Lett. 2010, 1, 21652173
Metal-Free Carbon Nanomaterials Become More Activethan Metal Catalysts and Last LongerDingshan Yu, Enoch Nagelli, Feng Du, and Liming Dai*
Department of Chemical Engineering, Case Western Reserve University, Cleveland, Ohio 44106
ABSTRACT Many reactions involve metals, especially noble metals or metaloxides as catalysts. Although metal-based catalysts have been playing a major rolein various industrial processes, they still suffer from multiple competitive dis-advantages, including their high cost, susceptibility to gas poisoning, and detri-mental effects on the environment. Owing to their wide availability, environmentalacceptability, corrosion resistance, and unique surface properties, certain carbonnanomaterials have recently been demonstrated to be promising metal-freealternatives for low-cost catalytic processes. This perspective highlights recentprogresses in the development of carbon-based metal-free catalysts.
G enerally speaking, a working catalyst must possesssurface active sites necessary for adsorption of thereactants, bond-breaking and bond-formation, anddesorption of the products. Additionally, an excellent structur-al stability is also essential to ensure that the catalytic activityis effective and efficient over a long period. Metals and metaloxides are undoubtedly the most widely used catalysts inmany industrialized catalytic processes. Pt, Au, and Ru are afew examples of the noblemetal catalysts used in fuel cells toaccelerate the oxygen reduction reaction (ORR) at thecathode,1-4whilemanyhydrogenation anddehydrogenationreactions involve metal oxides as catalysts.5,6 However, thesemetal-based catalysts often suffer from multiple competitivedisadvantages, including their high cost, low selectivity, poordurability, and detrimental environmental effects caused bycatalyst residues and/or undesirable side-products.7 There-fore, it is highly desirable to develop inexpensive, metal-freecatalysts of high performance.
Owing to their wide availability, environmental acceptability,corrosion resistance, and unique surface properties, carbonnanomaterials are ideal candidates for metal-free catalysts.While activated carbon and glassy carbon (GC) have been longused as catalysts for certain chemical and electrochemicalprocesses,8,9 the recent availability of carbon nanomaterials ofvarious peculiarmolecular structures andoptoelectronic proper-ties, including fullerenes, carbon nanotubes (CNTs), nanodia-monds, and graphene sheets, offer new opportunities for thedevelopment of advanced carbon-based catalysts with muchimproved catalytic performance.7,10-12 The introduction of sur-face heteroatoms (e.g., nitrogen) into these carbon nanomater-ials could further cause electronmodulation to provide desirableelectronic structures for many catalytic processes of practicalsignificance.7 Consequently, considerable effort has recentlybeen directed toward the development of metal-free carbonnanomaterials for various catalytic processes, involving either
oxidation or reduction reactions.7,13 This perspective high-lights recent progresses in the development of carbon-basedmetal-free catalysts with an emphasis on the use of CNTs foroxidative dehydrogenation (ODH) of aromatic hydrocarbonsand alkanes as well as ORRs, particularly in alkaline medium(Scheme 1).
Metal-Free Carbon Catalysts for ODH of Aromatic Hydrocar-bons and Alkanes. Conventional heterogeneous catalysts oftencontainmany active sites with a low free energy for chemisorp-tions, which also act as a matrix for a relatively small
Scheme 1. ODH of (1) Ethylbenzene and (2) Alkanes (a) and ORRin Alkaline (b) and Acidic (c) Media
Received Date: April 26, 2010Accepted Date: June 23, 2010
r 2010 American Chemical Society 2166 DOI: 10.1021/jz100533t |J. Phys. Chem. Lett. 2010, 1, 21652173
number of high-energy reactive sites for converting theadsorbedmolecules into products via bond rearrangements.14
For nanostructured carbon materials (e.g., CNTs, graphenesheets), the weak adsorption sites are normally associatedwith their basal planes consisting of the hexagonal arrange-ment of carbon atoms, while the high energy sites appearat the edges often saturated with hydrogen heteroa-toms. The edge hydrogen atoms can be replaced byother heteroatoms, for example, oxygen and/or nitrogenatoms, to provide strong chemical reactivities for redox oracid-base reactions. Along with its use as a support for pureand heterogeneous metal catalysis,15 carbon has alsobeen demonstrated to show interesting catalytic activitiestoward, for example, the ODH of saturated hydrocar-bons.16-21 In particular, a variety of nanostructured carbons,such as CNTs,17 carbon nanofibers (CNFs),18,19 carbononions,20 and nanodiamonds,21 have been found to effi-ciently catalyze theODH reaction of ethylbenzene to styrene(Scheme 1a). As an important monomer for polystyreneand its derivatives, styrene has normally been produced bythe direct dehydrogenation of ethylbenzene in the temp-erature range 560-650 C using iron oxide catalysts of alow energy efficiency.7 Compared with the traditionalmetal oxide catalyst,22,23 certain nanocarbons were demon-strated to exhibit a comparable, or even better, catalytic per-formance and long-term stability for the ODH reaction(Figure 1a) within the optimal temperature range of 350-550 C.17,18,20,24 The large surface area of nanostructuredcarbons, with and without functionalization, can providenumerous active sites, and hence the excellent catalyticactivity, while the graphitic structure can tightly hold theactive moieties for an excellent thermal stability evenunder an oxidative atmosphere. Although the presenceof oxygen-containing surface functional groups on thenanocarbon catalysts has been shown to be responsiblefor the catalytic ODH of ethylbenzene,24,25 the exact mecha-nism for the dissociative adsorption of gas-phase oxygen
molecules on carbon is still unknown and requires furtherstudy.
On the other hand, the activation of alkanes to alkenes hasattracted considerable interest during the past decades. Despiteconsiderable effort devoted to this important energy-relatedresearch field, the existing alkane activation process still suffersfrom the limited selectivity for alkene. Recently, Zhang et al.26
have demonstrated that simple oxidation of multiwalled CNTs(MWCNTs) by HNO3 increased the alkene yield from 1.6% to6.7% (Figure 1b). Subsequent passivation of MWCNT defectswith phosphorus oxide further increased the alkene yield up to13.8% with a comparable selectivity to the widely used V-Mg-O catalytic system.26 Similar catalytic activity enhance-ment was also observed with both the phosphorus- and boron-modified MWCNTs (P- and B-MWCNTs, respectively) for theODH of propane to propene.27,28 In this case, however, thepropene selectivity for either P-MWCNTs or B-MWCNTs is stilllower than that ofmetal oxide catalysts due to the relatively longproduct retention time causedby strong interactions of propenemolecules with the carbon catalyst surface. In addition, Meyeret al.,29 found that the selective adsorption of nitro-substitutedaromatics onto oxidized CNTs, fullerene, and even graphitecould accelerate the rate of hydrolysis of 4-nitrophenyl acetate
Figure 1. (a) Comparison of styrene yield from dehydrogenation reactions catalyzed by nanocarbons and metal catalysts (adapted fromrefs 17, 18, 20, 22, and 23). (b) Performance of variousMWCNTs forODHof butane under oxygen-rich conditions: 0.18 g, 0.67%butane, O2/butane =2, 15 mL min-1, 400 C (adapted from ref 26).
Compared with the traditional me-tal oxide catalyst, certain nanocar-bons were demonstrated to exhibita comparable, or even better, cata-lytic performance and long-termstability for the ODH reaction.
r 2010 American Chemical Society 2167 DOI: 10.1021/jz100533t |J. Phys. Chem. Lett. 2010, 1, 21652173
on carbon surfaces. MWCNTs have recently been shown to alsocatalyze the ODH of dihydroanthracene to anthracene undermild reaction conditions,30 while ordered mesoporous carbons(OMCs)werereported toactaspromisingcatalysts forhydrogenproduction via methane decomposition.31 However, furtherstudy on the reaction kinetics and mechanisms is still needed.
Metal-Free N-Doped Carbon Catalysts for ORR. N-doping ofCNTs has been studied for some years with attempts tomodulate the nanotube electronic and other properties byintroducing nitrogen heteroatoms into the nanotubestructure.32,33 N-doped carbon nanostructures can be preparedeither by in situ doping during the nanocarbon synthesis orthrough post-treatment (i.e., postdoping) of preformed carbonnanostructures with nitrogen-containing precursors (e.g.,NH3).
34-38 Postdoping of carbon nanomaterials often leads tosurface functionalization only without altering their bulk proper-ties.35-38 In contrast, the in situ doping can incorporate nitrogenatoms into the entire carbon nanomaterials homogeneously.Examples forN-doped carbonnanomaterials from in situ dopinginclude thehighsurfaceareaporouscarbonswithahighnitrogencontent prepared by cyclotrimerization reactions of carbonitrilesunder isothermal conditions,39 and N-doped CNTs or CNFs syn-thesized by arc-discharge, laser ablation, or chemical vapordeposition (CVD)40-43 in the presence of appropriate nitrogen-containing precurs